Hollow core doors and solid core doors having compression molded door facings are well known in the art. Typically, the door includes a perimeter frame, which includes right and left stiles and top and bottom rails attached together end-to-end to form a rectangular frame. Interior major surfaces of door skins (also referred to in the art and herein as door facings) are secured to opposite sides of the frame, typically using adhesive and/or fasteners.
There are several known techniques for making door skins. For example, the door skins may be formed by pressing a planar cellulosic mat or a fiberglass composite material, such as sheet molding compounds, in a compression mold press. Alternatively, pre-consolidated blanks may be pressed in the mold press to form the door skins. Other molding techniques may also be practiced.
A pressed door skin may be a “flush” door skin with planar interior and exterior surfaces. Alternatively, a pressed door skin may be shaped to include one or more molded contoured portions, also known as ovalos. Typically, the contours are continuous structures that define a square, rectangle, circle, or oval when viewed from an elevational viewpoint, although the contours may define other shapes. The area within the continuous contours is typically planar and gives the appearance of an interior panel that has been formed by machine routing. The contours appear from the exterior side of the door skin as depressions extending inwardly into the exterior surface of the door skin, and appear from the interior side view point as protrusions extending from the interior surface away from the exterior side.
A cavity is defined by the frame and the interior surfaces of the opposing door skins. If left empty, the hollow cavity typically causes the door to be lighter than a comparably sized solid, natural wood door. Such an artificially light weight hollow core door is not desirable for many consumers who expect the feel and weight of the door to replicate solid natural wood. In addition, the sound and/or heat insulation provided by hollow core doors are typically less than may be desired or specified. Therefore, it is often desirable to use a core structure (e.g., one or more core pieces or core components) to fill the cavity. Such core-containing doors are generally known as solid core doors. The core structure is flanked on its opposite major surfaces by door skins, and is surrounded at its periphery by the door frame. To enhance the heat and fire preventive properties of the solid core door, a fire retardant may be incorporated into the core structure composition as an additive. Alternatively, the core itself may be a fire resistant material, such as made from pearlite, vermiculite or the like.
As described above, in the case of a solid core door having molded door skins with contours, the protrusions extending inwardly from the interior sides of the door skins into the cavity impinge upon the space available for the core structure. Consequently, the core structure is provided with a lesser thickness, at least in the regions of the cavity corresponding to the molded door skin contours, to accommodate the inwardly extending contour protrusions. U.S. Pat. No. 5,887,402 to Ruggie et al. describes a solid core door including a core component having its major surfaces machine routed to include recesses at regions of the core component corresponding to the location of the contours of the door skin. The contour protrusions on the interior sides of the door skins are at least partially accommodated or received in the recesses. The areas of the core component surrounding the recessed regions have a greater thickness than the recessed regions to fill the cavity regions between the planar portions of the door skins. U.S. Pat. No. 6,764,625 discloses molding a fiber/resin mat in a conventional press to include recesses corresponding to the configuration of the depressions of the door skins. U.S. Pat. Nos. 7,695,658, 7,998,382, and 8,341,919 disclose pressing a pre-consolidated mat in a mold cavity to form at least one recess that corresponds to the configuration of a depression of a door skin.
The present inventors have observed that the core recesses machined, molded, or otherwise formed in the door core structure to accommodate the door skin contours may adversely affect the heat and fire resistant properties of the resulting door. When a side of a door having the recessed core structure is exposed to fire, the flow of heat to the non-exposed side of the door is greater at the core recessed areas, where core insulation material has been removed to accommodate the molded contours of the door skin, than at the thicker surrounding areas of the core structure that have a greater thickness. Many localities and/or building owners may specify requirements of a “fire rated” door. As identified by the inventors, an increase in the heat resistance in the recessed areas of the door core may be desirable in order to allow paneled doors to be utilized in such applications. American National Standards Institute ANSI/UL-10C is one standard used for evaluating the fire resistance of doors.